The present invention relates to a Voltage Source Converter having at least one phase leg connecting to opposite poles of a direct voltage side of the converter and comprising a series connection of switching assemblies, each switching assembly having an electrically conducting plate member carrying a plurality of semiconductor chips each having at least a semiconductor device of turn-off type and a free-wheeling diode connected in parallel therewith said chips being connected in parallel with each other by each being connected by an individual conductor member to a said plate member of an adjacent switching assembly of said series connection of switching assemblies, a midpoint of said series connection forming a phase output being configured to be connected to an alternating voltage side of the converter and to divide the phase leg into an upper valve branch and a lower valve branch.
Such converters with any number of said phase legs are comprised, but they have normally three such phase legs for having a three phase alternating voltage on the alternating voltage side thereof.
Furthermore, it is pointed out that the present invention is not restricted to any type of Voltage Source Converters, but it comprises for example Voltage Source Converters of the type known through for example DE 101 03 031 A1 and WO 2007/023064 A1, which are normally called multiple-cell converters of M2LCs, as well as two-level converters having current valves controlled for alternatively connecting one of said poles to said phase output and NPC (Neutral Point Clamped) converters.
A Voltage Source Converter of this type may be used in all kinds of situations, in which direct voltage is to be converted into alternating voltage and conversely, in which examples of such uses are in stations of HVDC-plants (High Voltage Direct Current), in which direct voltage is normally converted into a three-phase alternating voltage or conversely, or in so-called back-to-back-stations in which alternating voltage is firstly converted into direct voltage and this is then converted into alternating voltage, as well as in SVCs (Static Var Compensator), in which the direct voltage side consists of capacitors hanging freely. However, the present invention is not restricted to these applications, but other applications are also conceivable, such as in different types of drive systems for machines, vehicles etc.
Thus, the present invention is not restricted to any particular voltage levels between said opposite poles of the direct voltage side of the converter, but these levels shall be that high that a comparatively high number of switching assemblies are to be connected in series for making it possible for them to together hold said voltage in a blocking state of said semiconductor devices.
These semiconductor devices are mostly IGBTs, but any type of similar semiconductor device of turn-off type is conceivable. However, for illuminating but not in any way restricting the present invention the case of IGBTs as semiconductor devices in said semiconductor chips will mainly be discussed hereinafter.
A switching assembly in such a Voltage Source Converter has a plurality of said semiconductor chips connected in parallel, the IGBTs of which are controlled simultaneously to be in a conducting or a blocking state and to share the current through the switching assembly when they are in a conducting state. Continued operation of the Voltage Source Converter has to be ensured would one IGBT fail. Extra switching assemblies, i.e. more switching assemblies than are needed for holding the voltage to be held thereby for taking the voltage between said two poles, are in a Voltage Source Converter of this type arranged in said series connection of switching assemblies, so that it would not constitute any problem if one of them would stop function as long as this does not stop the current through the converter.
One type of Voltage Source Converters has said individual conductor members connecting the respective semiconductor chip to a said plate member in the form of bonded wires, and in a case of a short circuit in one IGBT of such a chip all the current through the switching assembly will flow through this chip and the bonded wire connected thereto will burn through and the current will then jump to another semiconductor chip in parallel therewith. When all the semiconductor chips of the switching assembly have been consumed in this way operation of the Voltage Source Converter has to be stopped and the switching assembly failed has to be replaced.
It is also known to ensure the connection between the switching assemblies connected in series by the so-called press pack technique, which means that a compression spring is arranged between a said plate member and each individual semiconductor chip, and said at least one individual conductor member is in that case constituted by relatively thin flexible conductors, which however are designed to withstand a short circuit current flowing through a failing IGBT. However, after some period of time the current path through the IGBT will be interrupted due to temperature constrains thereon and jump to another semiconductor chip connected in parallel therewith. With this type of series connection of the switching assemblies it may take an acceptably long time before all semiconductor chips of a switching assembly have been knocked out and operation of the converter has to be stopped, since the dimensioning of the current allowed through the converter is chosen so that this will take many years.
Would, however, there be a wish to increase the current substantially for increasing the power transferable through the Voltage Source Converter the lifetime of a switching assembly after one IGBT has been short circuited and until a need to stop the operation of the converter occurs may be considerably shortened, and there would in such a case be highly desired to reliably create permanent short circuit across this switching assembly for avoiding the occurrence of a said operation stop.